This is more of a situational technicality since it isn't what the skill is supposed to do, but the answer is bone spirit. If bone spirit is cast a certain distance away (I don't know the distance or if you have to traget something) then by chance it may become "invisible," put in quotation marks because it looks like a Burning soul when he isn't attacking, just a light glow on the ground. Pretty nseaky if you ask me. : P